I don't know a lot about Lemmy's implementation but a difficult thing to deal with is how do you "rank" a post? Like you have a small community of a few active people, but there's federation with a massive community with lots of users - which posts are "better"?

Worse still, there's an inherent lag/delay with the federated posts, a post that was very active in the last hour might have only been federated to the server in the last 5mins - so what do you do, do you bubble up all those posts or ignore it because there's more recent and relevant things?

The kicker is that these decision points aren't instant either, any system that's doing this kind of ranking will have an algorithm as you describe, but that algorithm will take time to process all the data, while the data is coming in batches as each server federates with each other. It's a difficult problem to solve.